Universal joints transmit torque where there can be misalignment of two components. In a drilling operation, a drill bit is mounted to the end of a drill string. The drill string is rotated from the top of the string or by a motor at the bottom of the string, or both, to rotate the drill bit and advance the borehole. Universal joints are included in the drill string to accommodate eccentricity in the string. The eccentric rotation is converted into axial rotation in order for the drill bit to advance the borehole efficiently. Eccentricity may be initiated by a motor in the drive assembly that rotates the drill bit or by steering of the bit to change direction of the borehole or other operating conditions.
FIG. 1 is a schematic representation of a drilling operation 2 to advance a borehole 4. The drill string 6 comprises sections of pipe that are threaded together at their ends to create a pipe of sufficient length to reach the bottom of the wellbore 4. The drill string is rotated in the bore either by a motor 22 proximate to the bit 8 at the end of the drill string or by rotating the drill string or both simultaneously. The drill string may be several miles long. A pump circulates drilling fluid through the drill pipe and out of the drill bit flushing rock cuttings from the bit and transporting them back up the wellbore. Additional tools can be added to the drill string for specific functions such as vibrating the drill string to reduce friction, downhole measurement or steering.
Mud motors are often used at the lower end of the drill string to rotate the bit. The motors have a rotor in a housing that rotates in response to the flow of drilling fluid. Universal joints are generally used to link the motor to the drill bit and convert eccentric rotation of the mud motor shaft to axial rotation. The constant relative movement of the components of the universal joint, in abrasive drilling mud, causes abrasion and erosion of mating components and can limit operational life of drill string components.